1. Field of the Invention
Disclosed embodiments relate to a multi-cylinder engine particularly provided with a crankcase integrated cylinder block.
2. Related Art
For a multi-cylinder engine, particularly, a multi-cylinder four-stroke cycle engine mounted in a motorcycle, in order to reduce the number of components or assembling steps, there is known a multi-cylinder engine including a crankcase integrated cylinder block in which a cylinder block including a cylinder bore and an upper crankcase that forms an upper half of a crank chamber are integrally formed.
Such a multi-cylinder engine including a crankcase integrated cylinder block includes a bulkhead (or partition wall) that partitions between adjacent cylinder bores and crank chambers, and the bulkhead has a communication hole that provides communication between the adjacent crank chambers. The communication hole acts to reduce pump loss caused by compression of air in a crank chamber in a piston downward-moving process or to reduce resistance to stirring caused by oil splashed in the crank chamber and mixed with air (for example, see Patent Document 1 (Japanese Patent Laid-Open Publication No. 2005-69170)).
In order to reduce a weight of an engine, a crankcase integrated cylinder block is formed of an aluminum alloy by casting. In this case, a cylinder inner wall surface, that is a slide surface with respect to a piston and forms a cylinder bore, is plated with metal such as nickel to prevent adhesion wear to the piston.
A method of plating the cylinder inner wall surface includes a so-called dipping method performed by dipping the entire crankcase integrated cylinder block in a treatment tank, and a method of blocking a crank chamber side end of a cylinder bore with a jig, and passing a plating solution from a cylinder head side of the cylinder bore (for example, see Patent Document 2 (Japanese Patent Laid-Open Publication No. 8-261055)). The method of blocking one end of the cylinder bore consumes smaller amounts of various plating solutions and requires less treatment time than the so-called dipping method.
The method of blocking one end of the cylinder bore to plate the cylinder inner wall surface described in Patent Document 2 requires a seal surface, which is not required in the so-called dipping method. The seal surface is a surface against which the jig to block the crank chamber side end of the cylinder bore is abutted, and a surface for keeping the inside of the cylinder bore liquid-tight so as to prevent the various plating solutions from leaking from the cylinder bore.
The crankcase integrated cylinder block includes a honing release portion having a honing release surface with a diameter larger than the cylinder bore continuously with the crank chamber side end of the cylinder bore. The honing release surface is a surface for releasing a honing edge of a honing machine in honing a cylinder bore, and used as a seal surface in plating the cylinder inner wall surface.
The crankcase integrated cylinder block has a communication hole that provides communication between adjacent crank chambers. The communication hole is positioned closer to a crank chamber than a lower end of a piston ring or a piston skirt at the lowermost stage of the piston at a bottom dead center in a boundary between the cylinder bore and the crank chamber, that is, near the honing release surface.
Thus, if an opening diameter of the communication hole is simply increased to further the reduce pump loss or resistance to stirring, the communication hole divides the honing release surface, and the method of blocking one end of the cylinder bore to plate the cylinder inner wall surface cannot be applied.
On the other hand, incidentally, the bulkhead partitioning the cylinder bore and the crank chamber also includes a bearing half portion that constitutes a journal bearing portion in which a crankshaft is journaled, and the engine includes a bearing cap that constitutes a journal bearing together with the bearing half portion of the bulkhead. The bearing cap is secured by fastening a fastening member such as a bolt in a fastening hole formed in the bulkhead. The fastening hole is, a so-called bolt hole, includes a female screw portion, and is placed adjacent to the bearing half portion.
That is, there is also known a crankcase integrated cylinder block having a rounded rectangular communication hole extending in a direction perpendicular to a cylinder axis, that is, a circumferential direction of a cylindrical surface that forms a cylinder bore (for example, see Patent Document 1).
For the communication hole extending in the direction perpendicular to the cylinder axis, a fastening hole for fastening a bearing cap to a bulkhead or a prepared hole thereof is placed close to an opening edge of a communication hole, which may reduce strength and durability against damage that occurs in the fastening hole or the prepared hole.
The bulkhead also includes a bearing half portion that constitutes a journal bearing portion in which a crankshaft is journaled, and the engine includes a bearing cap that constitutes a journal bearing together with the bearing half portion of the bulkhead. The bearing cap is secured by fastening a fastening member such as a bolt in a fastening hole formed in the bulkhead. The fastening hole is, a so-called bolt hole, includes a female screw portion, and is placed adjacent to the bearing half portion.
A crankcase integrated cylinder block has been known having a rounded rectangular communication hole extending in a direction perpendicular to a cylinder axis, that is, a circumferential direction of a cylindrical surface that forms a cylinder bore (for example, see Patent Document 1).
For the communication hole extending in the direction perpendicular to the cylinder axis, a fastening hole for fastening a bearing cap to a bulkhead or a prepared hole thereof is placed close to an opening edge of a communication hole, which may reduce strength and durability against damage that occurs in the fastening hole or the prepared hole.